The growth of aluminum nitride (AlN) bulk crystals by sublimation of an AlN source requires elevated temperatures, typically in a range of 1800° C. to 2300° C., with the upper end of the range yielding commercially viable growth rates of 0.1 to greater than 1 mm/hr. Reactor pressures are typically near or below one atmosphere, and the reactor headspace gas consists of pure nitrogen or nitrogen/hydrogen mixtures. The elevated temperature requirements, combined with the chemically aggressive nature of the Al vapor, severely limit the choice of reactor hot zone materials, and most notably, the selection of reaction crucibles.
The choice of crucibles for physical vapor transport (PVT) growth of AlN bulk crystals is dictated by several limiting requirements. Ideally, the crucible should be (1) refractory and compatible with the aforementioned growth temperatures, (2) inert to chemically aggressive Al vapor, (3) a negligible source of contamination to the growth process, (4) re-usable for multiple growth runs, (5) relatively inexpensive, and (6) manufacturable in various shapes and dimensions.
Plasma-spray processes have been used to form crucibles for crystal growth. The plasma-spray process involves spraying vaporized material onto a surface to provide a coating. A refractory material in the form of a powder is injected into a very high temperature plasma flame, where it is rapidly heated and accelerated to a high velocity. The hot material impacts on the substrate surface and rapidly cools to form a coating.
However, plasma-spray processes have several drawbacks. For example, plasma-spray processes do not allow precise machining of shapes before they are densified and rendered too hard for conventional metal machining tools. Further, plasma-spray processes are characterized by relatively low yields, which is problematic due to the high cost of most refractory materials, and are limited in the thicknesses and shapes that can be formed.
There remains a need in the art for new highly dense shaped articles and methods for preparing such articles that overcome the shortcomings of the prior art.